grep -E option is illegal on Illumos

[Chris Ridd]

Illumos's grep doesn't understand the -E argument. It is used in both scripts, here's the 2 calls in zfssnap.sh:

echo "$usrlabel" | grep -qiE '^([a-z0-9]{1,10})$' || failmsg "Invalid label '$usrlabel'! Quitting."
label_pfx="$label_pfx$usrlabel"

# maxnum
echo "$maxnum" | grep -qE '^[0-9]+$' || failmsg "Invalid maxnum '$maxnum'! Terminating."

Using /usr/xpg4/bin/grep might be better on this platform.